1. Field of the Invention
The instant invention is related to the automatic control of industrial processes. In a typical installation a process variable monitor or transmitter, such as a temperature probe, is used to generate a process value signal or PVS related to the present value of a process variable, such as temperature. A set-point generator provides a set-point signal or SPS which is equal to or related to the desired value of the process variable signal as determined by process requirements. An automatic process controller is used to generate a process control signal or PCS which is used to control a final operator intimately related to the process variable such as a flow valve operator. In this manner, the temperature of a liquid in the boiler, for example, may be controlled automatically by varying fluid flow through a valve.
This invention further relates to multi-functional process controllers of the type having both automatic and manual modes of operation.
In the automatic control mode of operation, a control circuit is used to generate an automatic control signal or ACS by applying a control algorithm, selected to fulfill process requirements, to a deviation signal or DS related to the deviation of the process value signal from the set-point signal. In this mode the ACS is used as the PCS to control the final operator to minimize the deviation of the process value from the set point. A typical control algorithm may require the ACS to be equal to or related to the sum of two or more terms or parameters including, for example, the DS multiplied by a proportionality or gain constant, the derivative of the DS called the rate term, and the integral of the DS called the reset term. These terms are usually adjustable by external means to vary the proportionality constant and the time constants of the rate and reset terms.
The set-point signal may be supplied by a local signal generator called the local set-point source or by a remote set-point source which may be the PCS of another controller arranged in cascade fashion or the output of a supervisory computer.
In the manual control mode an adjustable memory device called the manual memory is used to generate a constant manual control signal or MCS related to a value stored in the memory which may be adjusted by a process operator or process engineer according to process requirements. Manual memories may be constructed from analog components or, preferably, from a digital value storage device as described in our copending U.S. patent application Ser. No. 497,098 filed on even date herewith. Each patent application relates to different features of the same automatic process controller.
This invention still further relates to automatic process controllers for use with process installations in which an abrupt change in the process control signal is undesirable. Such abrupt changes in the PCS, called bumps, may be caused by transfer between modes, adjustment of the control parameters or switching between set-point sources. This invention therefore particularly relates to automatic process controllers providing automatically bumpless operation.
2. Description of the Prior Art
Early automatic process control installations utilized a non-automatic transfer system requiring a "balance" mode in order to achieve bumpless operation. In such systems an operator was required to physically adjust certain values during the balance mode in accordance with other values displayed by the controller to avoid bumps.
The conventional approach to providing automatically bumpless operation of a process controller is to provide bumpless transfer between automatic and manual modes of control. A bumpless transfer between automatic and manual modes of control. A bumpless transfer between automatic and manual modes occurs when ACS, PCS and MCS are all equal at the instant of transfer. The conventional approach therefore causes ACS to track PCS during the manual mode and MCS to track PCS during the automatic mode so that at the instant of transfer in either direction ACS = MCS. Automatic transfer between automatic and manual modes of control according to the above described philosophy is shown in U.S. Pat. Nos. 3,523,193; 3,550,014 and 3,655,992.
A major drawback of such approaches is that they provide automatically bumpless operation only for transfer between manual and automatic modes of operation. Switching between set-point sources and/or adjustment of the parameters cannot be accomplished bumplessly during the automatic mode of control with conventionally known systems.